The July 8 issue of the prestigious science journal Physical Review Letters (PRL) published an article that stirred up a controversy within the particle physicists’ community. A study conducted at Fermilab by international researchers, including four Brazilians, indicated that muon neutrinos and their respective antineutrinos possibly did not behave exactly in the same manner and might even have distinct masses. The study showed that the differences between matter and anti-matter might be bigger than those stated in the standard model, the theoretical framework built up over the last 50 years to explain the interactions between subatomic particles, the blocks that form matter. This was a surprising result, based on an analysis of the preliminary information obtained up to June 2010 by the Minos (Main Injector Neutrino Oscillation Search) experiment, one of the scientific projects under way at the American laboratory in Batavia, near Chicago. The content of the article, which apparently contradicts some of the laws of physics, according to the authors themselves, should be interpreted cautiously. There is a 2% chance that the unusual initial data produced by Minos was due to a momentary statistical fluctuation, therefore failing to mirror the reality of neutrinos and antineutrinos.
On August 25, after nearly doubling the quantity of information processed by the experiment in relation to the data contained in the article published in PRL, Fermilab made a public announcement. “More accurate measurements have shown us that probably these particles and their anti-particles are not as different as we had previously indicated. Within our current vision, it now seems that the Universe is behaving in the way most people expect it to behave,” said the announcement to the press made by Rob Plunkett, a scientist from Fermilab and one of the spokespersons for Minos. According to the study published in PRL, which had been confirmed by the traditional peer review before being accepted for publication, the square of the mass of the antineutrinos – the researchers used the value of the mass squared to the second highest power, and not only the measurement of the mass, as a comparison parameter – seemed to be approximately 40% higher than that of the neutrinos. “We spent nearly one year looking for some instrumentation effect that might have caused this difference. It is comforting to know that statistics were to blame,” said physicist Jenny Thomas, of University College London, another spokesperson for the experiment. According to the latest information reviewed internally by the researchers from Fermilab at the end of last month, but not yet submitted for peer review, this difference has dropped to 16%.
Therefore, it is very likely that the masses of neutrinos and antineutrinos are the same, as currently accepted physics models claim.
Brazilian physicist Carlos Escobar, one of the participants of Minos, explains that the review of the experiment’s results was submitted in order to avoid any kind of biased analysis. “Everything was conducted blindly and in an automated manner,” says Escobar, currently a collaborating professor of the State University of Campinas (Unicamp) and a researcher at Fermilab. “Data is sovereign.” However, he admits that the new scenario has relieved physicists. “The scientific community is more relieved now,” says Escobar. Several international experiments on particles and antiparticles have assumed that neutrinos and antineutrinos have the same mass when the related calculations were being done. When a study is published that contradicts such principles, as is the case of the Minos article printed in PRL, one of the pillars of physics stands to be affected.
Disappearance and oscillation
The objective of the Fermilab Project is to compare the occurrence of a phenomenon known as oscillation in the neutrinos and in antineutrinos of muon. In physics jargon, oscillation occurs when a type of neutrino or of antineutrino is transformed when it moves. There are three types or flavors of neutrinos and antineutrinos: those of muon, those of tau, and those of electron. This trio of electrically charged particles is referred to generically as leptons (neutrinos are neutral leptons). In the Minos experiment, scientists compared the frequency with which muon neutrinos and antineutrinos disappeared and supposedly turned into tau neutrinos and antineutrinos. “This is the first time that a group of researchers has measured the oscillations of muon antineutrinos,” says Philippe Gouffon, from the Physics Institute at the University of São Paulo (IF-USP), who also participates in the experiment conducted in the United States.
Conceptually, antimatter is defined as a mirror copy of matter, with which it basically shares the same properties, including mass. However, there is a basic difference between both: the electric charge of the antiparticles that shape antimatter have the opposite signal to their respective particles of matter. When it is positively charged, a positron is the antiparticle of the negatively charged electron. As their names indicate, neutrinos and antineutrinos are electrically neutral. However, neutrinos are linked to negatively charged leptons, whereas antineutrinos are linked to the positively charged ones. Physicists believe that matter and antimatter should exist in the same proportion in the Universe, even though the detected quantity of both is far from being equal. This is basically the theoretical context in which physicists study the properties of neutrinos and antineutrinos.
Even though they are considered the second most abundant particle in the Universe, right behind photons (particles of light), neutrinos are virtually imperceptible. They are not electrically charged; they have an insignificant mass, move at almost the speed of light, and barely interact with matter. They are able to go through enormous bodies, such as our planet Earth, without changing their movement or undergoing any noticeable effect. The Big Bang, the initial explosion that, according to the most widely accepted theory, created the Universe nearly 14 billion years ago, might have been the main source of neutrinos. Solar activity and cosmic rays are the best-known natural sources of neutrinos, which are formed by means of processes such as radioactive decay (when the nucleus of a stable atom spontaneously loses energy and emits ionized particles) and nuclear reactions.
Noise and information
It was possible to compare parameters between particles and antiparticles because the experiment conducted at Fermilab – together with Japan’s T2K (Tokai to Kamioka) experiment – was able to produce specific beams comprised only of neutrinos or only of antineutrinos, with minimum contamination levels. The majority of the scientific experiments use beams that mix particles and antiparticles, which makes it difficult to obtain detailed data on the oscillation phenomenon. “One of our greatest difficulties is to produce a system that generates enough particles to separate the noise coming from information,” explains physicist João Coelho, a PhD student at Unicamp. He spent one year at Fermilab thanks to a grant from FAPESP.
The first stage of the Minos experiment is to generate the particles that the physicists want to study. To this end, the Main Injector – a ring with a circumference of 3.2 kilometers and one of the six particle accelerators at Fermilab – produces a high energy pulse that later collides with a graphite target. The collision produces unstable particles – pions and kaons – which in turn will generate muons and neutrinos. Then the beams are guided to a wall that bars their impurities. Muons and other unwanted particles are removed, and only the muon’s neutrinos are left.
The second part of the experiment is the core of Minos. The purified neutrinos beam is guided to two underground detectors – the first lies one kilometer away from Fermilab and the other, 735 kilometers away, in the deactivated Soudan mine in the state of Minnesota. The closer detector, assembled approximately 100 meters lower than Fermilab, weights one thousand tons. This detector checks the purity and intensity of the beam. The detector’s measurements provide the main characteristics of the pulse. The more distant detector weighs 6 thousand tons and is buried 716 meters beneath the ground, in a cave. A mere 2.5 milliseconds after leaving Fermilab, the neutrino beam is detected in Soudan. “The neutrinos’ oscillations occur during the particles journey from the first detector to the second,” explains physicist Ricardo Gomes, from the Federal University of Goias, who is also taking part in the Minos project.
When they first measured the disappearance of the muon’s antineutrinos, the scientists from Fermilab initially thought that the oscillations of these antiparticles might be distinct, as assumed in the article published in PRL. Now that more data has been analyzed, the team working on Minos believes that this parameter is the same for neutrinos and antineutrinos. “Physics is essentially an experimental science,” says Marcelo Guzzo, a theoretical physicist from Unicamp who studies neutrinos. “Any result has to be confirmed by various groups before we get a definitive result,” says Orlando Peres, another expert on neutrinos who is also from Unicamp. According to physicist Renata Zukanovich Funchal, from USP, statistical fluctuations occur frequently in experiments with high energies: “This is why we have to be very careful when we find results that we are unable to understand.”
Adamson, P. et al. First direct observation of muon antineutrino disappearance. Physical Review Letters. v. 10 (2), p. 021801-5. July 5, 2011.